Doug Fleenor Design DMXOPTO-5 - Manual (Page 8)

7
LINE TERMINATION
Line Termination refers to the electrical properties at the end
of a control cable. In analog, and analog multiplex systems,
line termination is less important than it is in high speed
digital applications. Termination always improves DMX512's
signal quality. In some simple cases it can be ignored with
little loss of reliability. Line Termination becomes more
important as cable length increases, and as more dimmers
(receivers) are connected to the line. In complex systems, with
dimmers separated by more than a few feet, termination is
recommended.
Termination is provided by connecting a resistor between the Data
True (+Data) and Data Complement (-Data) lines at the very end of
the control cable run (to the control out connector of the last
dimmer). To be technically accurate, the value of the resistor
should match the "characteristic impedance" of the wire used for
control wiring.
This "characteristic impedance" is often not known for the wire
being used as control cable. A common impedance for RS-485
digital control cable is 120 Ohms. Even if the control cable
used is not exactly 120 Ohms, terminating the control line with a
120 Ohm resistor is better than leaving it unterminated (at
22,000 Ohms).
A simple "terminator" may be made by soldering a 120 Ohm 1/4 Watt
resistor between pins 2 and 3 of a 5 pin "XLR" style connector,
and plugging it into the last dimmer in the chain. It is
suggested that the connector be wrapped with white electrical
tape and labeled "DMX LINE TERMINATOR" for easy identification.
If you don't like soldering, a professionally constructed
terminator is available from Doug Fleenor Design.
Editor's Note: Since this manual was first written in 1990,
DMX512 systems have become more and more elaborate. Doug Fleenor
Design now recommends termination of all DMX512 systems,
regardless of complexity. Terminators cost less than $25.00.
Why risk failure?